The invention relates generally to the field of suppression of acoustic noise and relates more specifically to muffling acoustic noise in ducted or piped systems having a fluid flowing therethrough.
Systems having a volume flow throughput are common. They include: air-conditioning, heating and ventilating, steam power, internal combustion engines, vacuum cleaners, hair dryers, furnaces, gas turbines, positive displacement compressors, etc. Many of these systems share a common problem: they are acoustically noisy, and thus disturbing. The noise derives from perturbations in the fluid flowing through the ducting system at a frequency which gives rise to acoustic vibrations, i.e., vibrations within the audible range, of approximately 20 Hz to over 20 kHz.
The noise is a problem for many reasons. It is annoying to people who are near to the system. It may acoustically pollute an environment that requires relative quiet for other reasons, such as observation of acoustic phenomena. It renders it difficult for a vehicle including such a system to be used for surveillance, intelligence gathering, or other stealthy operations. It renders travel in such a vehicle unpleasant and degrades the enjoyment of any audio system carried thereby. The acoustic vibration may itself set off other vibrations at other parts in the system, which can mechanically disrupt elements in the system.
One type of conventional muffler, typically known as an "expansion" muffler, is shown schematically in FIG. 1. The transmission loss LT for an acoustic wave incident on an acoustic element is defined as: ##EQU2##
where, I.sub.i is the intensity (in units of pressure times velocity) of an acoustic sound wave incident on the acoustic element and I.sub.t is the intensity of an acoustic sound wave transmitted through an acoustic element.
The transmission losses LT achieved by a conventional expansion chamber muffler, having an inlet (and exit) duct cross sectional area of A.sub.1 and an expansion chamber cross sectional area of A.sub.2 is given by: ##EQU3## where m=A.sub.2 /A.sub.1, .omega.=the frequency of the acoustic vibration, in radians per second, c is the speed of sound in the acoustic medium and L is the length of the expanded chamber.
The relationship between the transmission loss (in dB) and the frequency of an acoustic sound wave for a typical expansion chamber type muffler is shown graphically in FIG. 2. Two different values for m are shown, with the generally upper trace being for m=16, and the lower trace for m=4. The other parameters used for FIG. 2 assume L=2 ft (0.61 m) and c=1100 ft/sec (279.4 m/sec), typical of atmospheric conditions. As is shown, with other things being equal, a larger m results in a relatively larger transmission loss.
Such an expansion muffler becomes ineffective at low frequencies, where the wave length (.lambda.=2.pi.c/.omega.) of the acoustic oscillations is large compared to the length L of the resonator. In air at atmospheric conditions, the wavelength for a 10 Hz oscillation is approximately 100 ft. As such, a conventional muffler would have to be approximately 25 ft in length to muffle efficiently at this frequency.
This limitation makes the expansion chamber muffler impractical for attenuating low frequency noise in many applications, since the required dimensions of the muffler become prohibitively large. This limitation also limits the effectiveness of a Helmholtz resonator muffler for attenuating low frequency noise for the same reasons. As shown, the expansion chamber also becomes ineffective at frequencies equal to n.pi.c/L, where n is an integer. For the case shown, these frequencies are multiples of 275 Hz.
Expansion and Helmholtz resonator mufflers also suffer from the problem of introducing a potentially undesirable pressure drop into the ducting or piping system in which they are installed. This results in a reduction in the efficiency or power delivery capabilities of the subject system.
Another undesirable feature of conventional type mufflers is the gradual slope of the transmission loss vs. frequency curve, between a zero transmission loss and the maximum transmission loss. This gradual slope means that there is a relatively large frequency range over which the transmission loss is less than the maximum. Such a situation is undesirable if the lowest desired frequency to be muffled is close to zero Hz. However, it is also not desirable that the transmission loss be at a maximum for a frequency range that extends all the way down to zero Hz.
It would be possible to muffle even low frequency noise (down to essentially 0 Hz) using a device such as a screen of an appropriate mesh. However, there is a relation between transmission loss at zero Hz and steady state pressure drop. Such a device would have a large pressure drop across it at steady state. This is undesirable.
A more desirable scenario, where the lowest frequency to be muffled is, in fact, very low, is where the transmission loss is zero for steady state and very low frequency perturbations, and also that the transmission loss abruptly rises from a low to a high value, transitioning from a low to a high transmission loss over a relatively small frequency range. As such, the curve relating transmission loss to frequency would substantially assume the form of a step function. In situations where the lowest frequency of noise desired to be muffled is not very low, it is not so important that the transition be abrupt.
Thus, an object of the invention includes to suppress a wide bandwidth of acoustic vibration in conduit systems, including but not limited to pipe or duct systems having a volume flow, including low frequency vibrations. It is a further object of the invention to suppress such vibrations with an apparatus that is not unduly large in the context of subject system. A further object of the invention is to suppress such vibration while minimizing the pressure drop across any muffling element or otherwise significantly reducing the efficiency or maximum power delivery capability of the system being silenced. Another object of the invention is to suppress noise with a device exhibiting a transmission loss to frequency relation that has a minimal or zero transmission loss at frequencies near steady state, and a transition to a high transmission loss. In some circumstances, it may be desirable that the transition from zero to high transmission loss approximate a step function, encompassing a relatively narrow range of frequencies.